"most likely" is correct and usable in written English. You can use it when you are expressing the likelihood of something happening, such as: "Most likely, the rain will clear up by lunchtime.".
Most likely.
"Most likely a hawk.
Most likely neither.
Most likely dead.
Most likely in Texas.
Most likely the latter.
Most likely, he'll be back.
Awesome tool! I started using it one year ago and I never had to look for another app
Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia